1. Field of the Invention
The present invention relates to a process for producing a high-purity polycrystalline silicon ingot to be made into solar cells. More particularly, the present invention is concerned with a process for producing high-purity silicon for solar cells continuously directly from inexpensive silicon containing a comparatively large amount of impurities by remelting it.
2. Description of the Prior Art
The silicon substrate for solar cells should be of high purity so that it possesses the characteristic properties of semiconductor required of solar cells. The level of purity should be such that the total amount of impurities such as B, P, Al, Fe, and Ti should not exceed 1 ppm. Moreover,-the silicon substrate should have grain boundaries aligned and defects (such as dislocation) reduced to a minimum. These requirements have conventionally been met by making an ingot by casting from expensive silicon of semiconductor grade and slicing it into substrates. This method suffers from several disadvantages, including high material cost, entrance of impurities from the casting mold, and poor productivity. (Casting needs a large amount of manpower and materials.)
To eliminate these disadvantages, there has recently been proposed a purification process by high-frequency plasma for the production of high-purity silicon(as disclosed in Japanese Patent Laid-open No. 218506/1988). This process, however, still has a disadvantage of low yields and necessity for complex post-treatment, because melting for purification is accomplished partly, with the remainder in contact with the container, to avoid the entrance of impurities from the container.
Also, there has recently been proposed an improved casting method which utilizes electromagnetic power for melting (as disclosed in Japanese Patent Application No. 167194/1988). This method permits the removal of heavy metals (such as Fe and Ti) by segregation, but it does not effectively remove B and P which have a high segregation coefficient (partition ratio of a substance between the solid and liquid phases). Moreover, this method needs semiconductor grade silicon as a raw material.